The present invention relates to insulator supports for electrically insulating an axially extending support rod carrying an electrical charge from a grounded housing surrounding the support rod and, more particularly, to an improved cap structure for such a housing which facilitates maintainance of the interior of the housing in a clean condition free of excessive dust accumulation. The invention has particular applicability to insulators used for supporting the discharge electrode hanger rods from the grounded housing of an electrostatic precipitator.
In the operation of an electrostatic precipitator, a gas laden with entrained particulate material is passed through an electrostatic field established about a discharge electrode disposed between two grounded collecting electrodes. The suspended particles become electrically charged as they pass through the electrostatic field and move under the influence of the electrostatic field to and deposit upon the electrically grounded collecting electrodes flanking each discharge electrode. Each collecting electrode is typically formed of one or more elongated plates disposed and suspended from the top of the precipitator housing in a vertical plane. A plurality of such collecting electrodes are disposed transversely across the precipitator casing in spaced vertical planes parallel to the direction of gas flow through the precipitator.
In the most common electrostatic precipitators, referred to as rigid frame electrostatic precipitators, a boxlike framework comprised of a plurality of discharge electrode frames mounted in a framework which is suspended by hanger rods from support insulators at the top of the electrically grounded precipitator housing to provide a row of vertically disposed discharge electrodes between adjacent collecting electrodes across the width of the precipitator. The voltage is applied to the discharge electrodes to generate the electrostatic field. Each discharge electrode frame is comprised of a plurality of individual flexible discharge electrode members, commonly comprising wires tautly strung across the support frame or tubes extending between frame members.
Since the hanger rods supporting the discharge electrode mounted to the framework are mounted to the grounded precipitator housing and since the hanger rods are typically electrically conductive, each hanger rod supporting the electrically charged discharge electrode framework is insulated from the grounded precipitator housing by an electrically non-conductive can-like insulator disposed coaxially about an opening in the precipitator housing through which the hanger rod extends coaxially through the top of the insulator housing from which it is supported in electrical isolation from the grounded precipitator housing.
Since the insulator housing remains in fluid communication with the interior of the precipitation chamber, particulate matter may settle in the insulator housing and accumulate sufficiently to bridge the annular space from the hanger rod to the interior surface of the region of the precipitator housing surrounding the opening through which the hanger rod passes, thereby establishing a conductive path along which electrical arcing may occur. One common method applied to attempt to limit such adverse dust accumulation within the insulator housing is to provide a flow of higher pressure cleaning gas, such as an inert gas, air or heated air, into the interior of the insulator housing to flow outwardly therefrom into the precipitation chamber thereby preventing particulate entrained gas from entering the insulator housing to deposit particulate matter therein, as well as providing a continuous purging airflow to carry particulate matter that may have been deposited in the insulator housing into the precipitator housing. Examples of such a design are disclosed in U.S. Pat. Nos. 3,531,918, 4,294,591; and 4,578,088. The scavenging airflow introduced into the insulator may be heated as disclosed to U.S. Pat. No. 4,294,591 to exceed the dewpoint temperature of gas in the precipitator thereby preventing condensation of water vapor or acidic vapors within the insulator housing.